1. Field of Invention
The present invention relates to a technical field of power electronics. More particularly, the present invention relates to a three-phase AC (Alternating Current)-DC (Direct Current) converter circuit for a power converter, a conversion method of the three-phase AC-DC converter circuit and a control system of the three-phase AC-DC converter circuit.
2. Description of Related Art
With the development of power electronic technology, requirements on the harmonic content in the input current of a power electronic converter are becoming higher and higher. Taking a three-phase power factor correction (PFC) circuit as an example, a harmonic wave of the input current of the three-phase PFC circuit is mainly generated from a current ripple caused by switching a high-frequency switching component. Thus, in order to reduce the harmonic wave of the input current, while switching frequency of the high-frequency switching component is maintained unchanged, a conventional solution increases the inductance in the three-phase PFC circuit or improves filtering effect at the EMI (electromagnetic interference) step, so as to reduce the current ripple. However, the inductance is increased by increasing the inductor's volume in the PFC circuit or in the EMI filter.
On the other hand, in a power electronic converter, taking the reliability of a circuit and the simplicity and convenience of its production into consideration, a multilevel circuit with a cascade structure is often used, thus the cascade structure almost has a DC bus, and a large electrolytic capacitor is often connected to the DC bus in parallel. For example, a common communication power supply module generally uses a two-stage structure with a DC bus. In the two-stage structure, a pre-stage circuit is often a PFC circuit, and a post-stage circuit is often a DC-DC conversion circuit. When the pre-stage circuit and the post-stage circuit are in a stable state, the mean value of the input power is equal to that of the output power but the transient input power is different from that of the output power. Thus, the electrolytic capacitor only allows the alternating current to pass through, so as to balance the difference between the transient powers of the pre-stage and post-stage circuits.
However, the electrolytic capacitor occupies a large volume in the power converter, thus increasing the cost. Furthermore, due to technology characteristics of the electrolytic capacitor, the electrolytic capacitor has to reduce its temperature rise as much as possible during operation in order to prolong its operation life. In general, the temperature rise of the electrolytic capacitor is mainly caused by two factors. One is the AC consumption on the equivalent series resistance of the electrolytic capacitor, and the other is the impact of environment temperature and heat dissipation conditions on the electrolytic capacitor as well as the impact of other heating generating components on the electrolytic capacitor. In the prior art, for the heat-dissipation problem of the electrolytic capacitor, one solution is to increase the number or volume of the electrolytic capacitor so as to reduce the equivalent series resistance thereof and thus reduce the loss. However, the increase of the number or volume of the electrolytic capacitor accordingly increases the volume of the whole system, thus reducing the power density of the converter. Another solution is merely to improve the heat dissipation capability of the electrolytic capacitor, such as increasing the amount of wind passing through the system. However, if the system is operated under a high-temperature environment, the heat dissipation of the electrolytic capacitor is not effective.
In view of the above, those in the industry are endeavoring to find the way to design a novel power converter, so as to reduce the inductance of the power converter or reduce the temperature rise of the electrolytic capacitor, or increase the power density of the power converter.